Add a method to create an AttributeSet from an AttrBuilder.

The Attribute class is eventually going to represent one attribute. So we need
this class to create the set of attributes. Add some iterator methods to the
builder to access its internal bits in a nice way.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171586 91177308-0d34-0410-b5e6-96231b3b80d8